The present invention relates to a new method concerning the environment protection, specifically to a new method for the biological purification of wastewater from domestic sewage and industrial wastewater, e.g. from chemical or oil refining plants, with no excess activated sludge. It further relates to an apparatus for the performance of said new method.
The basic principle of biological purification of wastewaters resides in the joint conversion of the dissolved organic and/or inorganic matter (substrates) into the form of sludge (activated sludge), which has to be eliminated from the system, depending upon the chosen technical solution. There exist two basic types of biological purification of sewage with regard to the characteristics of the obtained sludge:
a) the biological treatment with activated sludge, in which the sludge is removed by clarification; subsequently to the precipitation the sludge is subjected to incineration or to aerobic or anaerobic digestion; PA1 b) the biological treatment in lagoons, where the obtained sludge exhibits dispersing instead of precipitating characteristics and requires the following elimination measures: PA1 a) for energetic needs, responding to the reaction equation: EQU substrate+O.sub.2 .revreaction.CO.sub.2 +H.sub.2 O PA1 b) for the synthesis of new biomass (new microorganisms) in the system as represented by the equation: EQU substrate+NH.sub.3 .revreaction.biomass PA1 c) for the so-called endogenous respiration by means of which a balanced state of the system is achieved, as represented by the equation: EQU biomass+O.sub.2 .revreaction.CO.sub.2 +NH.sub.3 +H.sub.2 O PA1 F=quantity of the substrate PA1 X.sub.V,a =quantity of the suspended matter in the biological basin PA1 S.sub.o =influent biological oxygen demand in 5 days PA1 Q=flow rate of wastewater PA1 M/F=0-1--short term aeration (highly loaded aeration) PA1 M/F=1-5 --conventional aeration with or without denitrification PA1 M/F=5-20--extended aeration PA1 M/F=20-100 --lagoon PA1 The short term aeration consumes the substrate predominantly for energy needs, whereas a substantial part of the substrate remains non-degraded. PA1 The conventional aeration with or without denitrification represents a very often applied principle of biological purification, wherein the part of the non-consumed substrate is very low (&lt;5%), whereas the consumed substrate is substantially divided between the energy demand and the consumption for the synthesis of new biomass. The quantity of the formed sludge is essential in such a treatment because the endogenous respiration is less expressed. For this reason it is believed that the conventional aeration yields an effluent of good quality; there exists, however, the basic problem of large quantities of the of the formed sludge to be processed. PA1 Extended aeration represents such a type of biological treatment of wastewater, wherein the endogenous respiration is the dominating feature, which is a result of the increased M/F value (M/F=5-20) and of the reduction of the F component in this ratio. This means that the quantity of the substrate supplied to the system is lower than required for the balance of the system, which is achieved by the consumption of the cytoplasm of the neighbor biomass (the so-called "cannibalism principle"). The quality of the effluent of such a system is satisfactory, like in the previous system, the formed sludge, however, has poorer settling performances. PA1 In the lagoon (M/F&gt;20) the situation is essentially different owing to the empirical fact that the activated sludge floccules only possess precipitating properties within the range of M/F=1-2, whereas in the range of M/F&gt;20 they gain dispersional properties. PA1 i) The only satisfactory effluent parameter is the concentration (BOD.sub.5).sub.solv. (BOD=biological oxygen demand). PA1 ii) The (BOD.sub.5).sub.total, which is the sum of (BOD.sub.5).sub.solv. +(BOD.sub.5).sub.susp., is approximately equal in the influent and in the effluent (or even larger in the effluent). This results from the fact that in the lagoon conditions (BOD.sub.5).sub.solv. is, during the biological consumption of the substrate, converted into (BOD.sub.5).sub.susp., which is discharged from the system by the effluent because the sludge flocks possess no precipitating but dispersing properties. Owing to obtaining a low-quality effluent, lagoons do not serve as independent facilities for the biological treatment of wastewater but are combined with facultative or similar lagoons. PA1 iii) The share of the endogenous respiration in the lagoon is minimal and negligible, which is also one of the results of the dispersing characteristics of the sludge (graph in FIG. 1). PA1 diffusion type PA1 surface turbine type PA1 surface propeller type PA1 submersed turbine type PA1 a) the type of the biological basin PA1 b) the characteristics of the wastewater subjected to purification: PA1 The selection of the aerator should comply with the data of the following Table 3. PA1 In the design of the biological basins the depth of the basin is not an essential parameter for surface aerators (provided the mixing is complete and the biomass is kept in suspension - Table 3), whereas the surface of the basin is an essential parameter; in submersed aerators it is just the contrary. When using surface aerators, the surface of the basin should be round (if there is only one provided) or square if there are several of them. When using diffusion aerators, the surface should be in the form of an elongated rectangle (the volume should be in the form of a channel). PA1 The ratio between the aerator power and the basin volume should at minimum fulfill the data of the following Table 4, otherwise the system is not in suspension at all. PA1 the energy needs (N), PA1 the endogenous respiration (N, P), PA1 the generation of new biomass (N, P). PA1 a) In short term aeration, in addition to the discharge with the excess sludge, a part of the nitrogen is also lost via the effluent as a nitrate and the slightly increased amount of suspended matter which comprises the nitrogen in the form of aminoacids (protein). PA1 b) In the conventional aeration without denitrification a part of nitrogen is also lost via the effluent in the form of nitrates in addition to the excess sludge; the loss via the suspended matter in the effluent, however, is minimal. PA1 c) In the conventional aeration with denitrification there occurs the lowest content of nitrogen in the effluent of all described biological principles of wastewater purification, which is a result of the denitrification of nitrogen from the nitrate into the gaseous nitrogen (N.sub.2), which leaves the basin at the surface. This, however, does not mean that the consumption of nitrogen is not equal to the amount consumed in the conventional aeration without denitrification. PA1 d) In the extensive aeration the nitrogen is lost via the effluent in the form of nitrate and in the form of aminoacids (proteins) owing to the slightly increased concentration of the suspended matter in the effluent. The amount of the nitrate in the effluent in the extensive aeration may be such that the pH value of the effluent drops within the acidic range. PA1 e) Lagoons yield sludge of dispersing properties, accordingly they lose the predominant part of the nitrogen in the effluent in the form of suspended matter (aminoacids and proteins) and the residue in the form of nitrates. PA1 direct disposal PA1 disposal subsequently to solidification PA1 aerobic digestion PA1 anaerobic digestion PA1 incineration PA1 the M/F (microorganism to food) ratio is adjusted within the range of 20-100, characteristic of the sludge of dispersing properties; PA1 the excess sludge generated in the biodegradation of the substrate is inhibited to leave the biological basin by the exclusive use of surface turbine type aerators of a power of 5.10.sup.-2 to 12.10.sup.-2 kW/m.sup.3 of the basin; PA1 the level of the treated charge in the basin must be at least 5 m from the bottom of the basin; this feature is essential because the total mixing of the charge/activated sludge is impeded.
oxidation ponds, PA2 mechanical operations such as filtrations through sand filters or screening systems; PA2 the type of the aerator (Tables 2, 3) PA2 the design of the biological basin PA2 the ratio of the aerator power/volume of the biological basin PA2 the temperature of the water PA2 the biological loading of wastewater PA2 the concentration of biomass in the system PA2 the surface action of the wastewater
all these installations are located after the basic purification lagoon.
Literature: Water and Sewage Works (1971), Reference Number, p. (R-7)-(R-14); (R-18)-(R-22); Water and Sewage Works (1964), p. 295-297; Hydrocarbon Processing, Oct. 1979, p. 99-106.
The hitherto applied principle of elimination of the (dissolved) substrate from wastewater by means of biological treatment is represented in Graph 1. It can be concluded therefrom that the substrate dissolved in wastewater is consumed by the miroorganisms in three ways, which are in mutual dynamic balance, as follows:
Since all three above manners of consuming the substrate are in dynamic balance, the domination of any one of them depends upon the chosen conditions in the performance of the biological treatment of wastewater. The basic criterion of the domination of any of said exploitations of the dissolved substrate is the empirically assessed ratio M/F (microorganisms to food ratio), which defines the quantitative ratio of the biomass and the substrate in the system and is expressed by the following mathematical equation: ##EQU1## wherein M=quantity of the biomass
In practice, the typical M/F ratios are comprised within the values of 1-50. Depending upon their value, the biological wastewater treatments can be divided into:
In Graph 1 there is represented the exploitation of the substrate in the above-defined types of biological treatment of wastewater and it can be concluded therefrom:
The results are evident from Graph 2, as follows:
Oxygen Demand:
Each of the above-defined basic methods for the biological treatment of wastewater requires oxygen, which is consumed in energy yielding reactions and in endogeneous respiration reactions. The oxygen may be provided from air in molecular state (for energy needs and for endogenous respiration) or from the nitrate type compounds (for endogenous respiration).
In practice there is used the technical solution of charging air into the biological basins by means of aerators of various design. The aerators function as suppliers of oxygen (aeration of wastewater) and as stirrers of the active sludge and wastewater to keep the complete system in a suspended state, which is very important as the activated sludge in the ratio of M/F=1-20 exhibits settling properties. In a lagoon of M/F&gt;20, the adjustment of the activated sludge in the suspended state is superfluous because, owing to the dispersed state of the activated sludge, it is--by virtue of its basic properties--in a suspended state.
The quantity of oxygen that has to be charged into the biological basin is divided between the air, needed for the energy demand and the air for the endogenous respiration. The quantity of air needed for the energy demand is proportional to the F value in the M/F ratio. Since the lower M/F ratio also means an increased amount of sludge in the system as well as its improved settling properties, it is evident that in this case a part of the oxygen (air) is consumed for the stabilization of the suspension state in the whole system.
In the lagoon (high M/F ratio) the amount of oxygen supply needed for the energy demand is the most reduced of all discussed embodiments for the biological purification of wastewater.
The demand of oxygen for endogenous respiration is the largest in the extensive aeration, only partially expressed in the conventional one and completely lacking in short term aeration and in lagoons, which is in accordance with the conclusions from the graph in FIG. 1.
In Table 1 there is represented the demand for oxygen and the function of oxygen in any of the cited systems for the biological purification of wastewater.
TABLE 1 ______________________________________ Oxygen function and demand depending on the type of the biological treatment of wastewater Oxygen consumption Relative the sludge in ratio of Type of completely for for the total biological mixed energy endogenous oxygen treatment conditions needs respiration consumption ______________________________________ short term .largecircle. 0.9 conven- 1.0 tional extended 1.2 lagoon .largecircle. .largecircle. 0.2 ______________________________________ maximum consumption 3/4 of the total consumption 1/2 of the total consumption 1/4 of the total consumption .largecircle. without consumption
For the aeration of biological basins the following types of aerators are used in practice:
In Table 2 there are presented the types of aerators depending on the type of the biological treatment of wastewater.
TABLE 2 ______________________________________ Types of aerators depending on the type of the biological treatment of wastewater Type of Type of aerator biological diffu- surface surface submersed treatment sion turbine type propeller type turbine type ______________________________________ short term x x conven- x x x tional extended x x lagoon x x ______________________________________
The criteria in the selection of the aerator type are more of an empirical than of theoretical nature. Since the aeration of biological basins is an essential economic parameter in the biological treatment of wastewater, the design of said systems always takes into account that the amount of the charged oxygen should be equal to or in a slight excess over the theoretically needed one. The oxygen demand (kg O.sub.2 /h) for each type of biological treatment can be calculated from the ratio of the endogenous and the energetic respiration. The selection of the type of the aerators and the number thereof can be assessed on the basis of the empirical parameter of the type "oxygen transfer efficiency" (EPK), which represents the ratio of the oxygen transferred from the aerator to the flocks of the activated sludge and the total supply of oxygen, expressed in a percentage ratio.
The EPK value depends on:
It has been shown in practice that a high "EPK" is achieved when the following criteria are fulfilled:
TABLE 3 ______________________________________ Types of aerators depending on the "EPK" parameter and the possibility of keeping the activated sludge in suspension "EPK" Keeping the active sludge in Type of aerator parameter completely mixed conditions ______________________________________ diffusion 0.7-1.8 good (regardless of the depth) surface turbine 1.8-2.4 0.3-5 m good type &gt;5 m bad surface propeller 1.8 1-6 m good type submersed turbine 1.2-1.8 good (regardless of the depth) type ______________________________________
TABLE 4 ______________________________________ Ratio of the aerator power/volume of the biological basin Type of biological treatment Aerator power (kW/m.sup.3) ______________________________________ short term 2.5 .multidot. 10.sup.-2 conventional aeration 2.5 .multidot. 10.sup.-2 extended aeration 2.5 .multidot. 10.sup.-2 lagoon 1.5 .multidot. 10.sup.-2 ______________________________________
The temperature of the wastewater ought to influence the "EPK" value, taking into account that the solubility and distribution of oxygen are different at different temperatures. Since, however, the wastewater is always aerated in a minimum excess of 20% over the value proportional to the loading and the supplied oxygen is immediately spent for the biodegradation of the substrate, the effect of the temperature is practically only important in low loaded wastewaters because aerators of lower power are used in such conditions.
The "EPK" parameters do not differ if there are various amounts of the biomass in the biological basin; a different amount of biomass in the same biological basin, however, means an increased oxygen demand because a larger amount of biomass always means an intensified endogenous respiration.
If there are surfactants present in the wastewater (detergents), there occurs a sharp drop of the "EPK" value which can drop to the zero value, provided that an increased concentration of the surfactants and the use of surface aerators are involved.
The aeration of wastewater in the biological treatment thereof represents an essential economical parameter of each such system, therefore in practice, the aeration is performed in such a manner that the minimum possible energy is consumed and, simultaneously, the criterion of keeping the active sludge suspended in water is fulfilled and the value of the "EPK" parameter is maintained as high as possible.
Examples for energy savings in the performance of aeration depending on the type of biological treatment are given in the following Table 5. It represents the relationships between the input energy and the unit volume of the basin and between the input energy and the unit volume of the biological loading of the wastewater.
TABLE 5 ______________________________________ The relationship between the aerator power and the unit volume of the biological basin and the unit loading for the basic types of biological wastewater treatments Power of the aerator per Type of biologi- volume of the biological loading cal treatment biological basin (kW/m.sup.3) unit (kW/kg BOD.sub.5) ______________________________________ short term 6.10.sup.-2 -8.10.sup.-2 0.17-0.66 conventional 4.10.sup.-2 -6.10.sup.-2 0.66-1.01 extended 2.10.sup.-2 -4.10.sup.-2 1.01-1.06 lagoon 1.10.sup.-2 -2.10.sup.-2 1.06 ______________________________________
The nutrient question:
The biological treatments of wastewater exploit the nutrients of the nitrogen (N) and phosphorus (P) types for the following:
In the technique of wastewater treatment it has been established that the nutrients ought to fulfill the following relationship: EQU BOD.sub.5 : N: P=100: (5-10): (0.5-1.5)
The consumption of nitrogen and phosphorus, however, also depends on the type of the biological treatment.
The nutrients on the basis of nitrogen may be organic nitrogen compounds, ammonia, urea etc., and nitrates, the phosphorus compounds are preferably on the basis of orthophosphates.
In all methods of biological wastewater treatment with excess activated sludge, after attaining the basic balance between the substrate and the nutrient, there should be added such an amount of the nutrient that it corresponds to the amount discharged by the excess sludge or, in a lagoon, to the amount of nutrients discharged in the form of (BOD.sub.5).sub.tot owing to the dispersion properties of the sludge. The function of phosphorus is very simple (it is only incorporated into the new biomass), whereas the function of nitrogen depends upon the dominating structure of the nitrogen compounds and the type of the chosen biological treatment, since according to the selected way of biological treatment nitrogen can be converted from one chemical form into another and leave the system by way of the effluent in various chemical forms.
In the following scheme there is represented the possible conversion pathway of nitrogen compounds regardless of the type of the biological treatment, and FIG. 3 indicates the principles of the conversion of nitrogen compounds depending upon the type of the biological treatment. ##STR1##
From the graphs a) - e) of FIG. 3 it can be concluded that:
In connection with the nutrient and especially the nitrogen problem it can be concluded that in all discussed types of biological wastewater treatments there should be achieved the basic balance for the degradation of the substrate by means of dosing nitrogen in any possible way into the influent if the latter does not contain sufficient amounts of nitrogen, e.g. in household sewage. The added amount of the supplied nitrogen should be approximately equal (economically) to the energy demand for the aeration of the biological basins.
The influence of the quantity of biomass in the system:
The various types of the biological wastewater treatments demand the provision of different amounts of the biomass in the biological basin and the clarifier and the discharge of different amounts of the excess biomass from the system. In practice the most often encountered parameter indicating the concentration of the biomass in the system is the so-called "mix liquor volatile suspended solids" concentration (MLVSS), which amounts to 70-80% of the total suspended matter in well-monitored systems.
In Table 6 there are presented the conventional concentrations of the MLVSS parameter in various types of biological treatments.
TABLE 6 ______________________________________ Concentrations of MLVSS depending on the type of biological treatment MLVSS concentration (mg/l) Type of in the in the in the biological treatment biological basin clarifier effluent ______________________________________ short term 1500-2500 8000-10000 100 conventional 2500-3500 10000-12000 20 extended 3500-5000 &gt;12000 40 lagoon 400-800 without 400-800 clarifier ______________________________________
As stated above, the different MLVSS do not simultaneously mean a different oxygen demand (the consumption of oxygen is proportional to the biological loading and the intensity of endogenous repiration and only indirectly depends upon the biomass amount in the system). Yet since a part of the MLVSS concentration represents the excess sludge, which has to be subsequently processed, in practice the biological treatment should be performed, as a rule, with the aim to minimize the excess sludge.
The amount of excess sludge depending upon the type of biological treatment is represented in Table 7. It is evident therefrom that the excess sludge is maximal at biological methods characterized by a lower M/F value, which means that these methods involve higher costs for the processing of the excess sludge.
TABLE 7 ______________________________________ The amount of excess sludge depending on the biological treatment Type of biological treatment Amount of excess sludge ______________________________________ short term 20-40 m.sup.3 /day conventional aeration 5-20 m.sup.3 /day extended aeration 0.2-5 m.sup.3 /day lagoon the sludge is dispersed, the effluent contains excess sludge ______________________________________
The problem of excess sludge can be conventionally solved in one of the following manners:
The direct disposal is, in fact, not a form of excess sludge processing and is being abandoned because of the toxic components of the sludge.
The disposal subsequently to solidification is feasible in some cases of wastewaters, e.g. in the wastewater of food processing, it cannot, however, be considered as a general principle for excess sludge processing.
The aerobic digestion represents a form of excess sludge processing, wherein the organic part of the sludge is oxidized with oxygen into carbon dioxide; owing to the energy consumption required for the aeration, it involves additional costs for the wastewater treatment plants.
The anaerobic digestion represents a form of excess sludge processing, wherein the organic part of the sludge is, in the absence of oxygen, reduced to methane, which can be exploited as an energy source in the wastewater treatment plants with the provision that there is a considerable amount of sludge. From the viewpoint of capital investment, the costs of anaerobic digestion exceed those of the aerobic digestion.
The incineration of sludges represents the most expensive type of excess sludge processing, yet it is the best way from the technological-technical point of view because it yields ash consisting of inert components.
The comparison of the main prior art processings for excess sludge disposal depending upon the capital investment and operation costs is represented in the Table 7. It is evident therefrom that the investment in such an arrangement may attain depending upon the selected type --40-60% of the capital investment, whereas the operating costs may attain 30-40% of the total operating costs; therefore it is understandable that the excess sludge is desired to be minimized. It has to be mentioned that in practice quite often no department for sludge processing is foreseen in wastewater treatment plants owing to the lack of investment capital.
TABLE 8 ______________________________________ The comparison of the investment and operating costs in the excess sludge processing Capital investment for Type of excess excess sludge treatment Operational costs sludge processing (total = 100%) (total = 100%) ______________________________________ aerobic digestion 40% 40% anaerobic digestion 50% 30% incineration 50-60% 40% ______________________________________
Microorganisms involved in the biological wastewater treatment pertain to a heteropopulation comprising the following genera: Pseudomonas, Achromobacter, Micrococcus, Bacillus, Alcaligenes, Escherichia, Flavobacterium, Nocardia, Zoogloea, and Serratia.